1. Field of the Invention
This invention relates generally to a novel lead zirconium titanate (PZT) material having unique properties and application for PZT thin film capacitors, as well as to a deposition method for forming PZT films of such material, and ferroelectric capacitor structures employing such thin film material.
2. Description of the Related Art
There is a major effort by semiconductor companies throughout the world to commercialize high dielectric constant and ferroelectric thin films in advanced dynamic random access memories (DRAMs) and ferroelectric random access memories (FeRAMs), respectively.
While the majority of current efforts are directed to the commercial development of relatively large capacitors (e.g. 5 .mu.m.sup.2 area), the ultimate goal is to adapt ferroelectric random access memory technology for future generations of integrated circuit devices in which capacitor areas, cell sizes and operating voltages are scaled downward as the technology evolves.
For FeRAM devices, most research is currently being directed to either ferroelectric SrBi.sub.2 Ta.sub.2 O.sub.9 (SBT) or Pb(Zr,Ti)O.sub.3 (PZT) thin films. Each material has relative advantages and disadvantages. Pt/SBT/Pt capacitors, for example, have been shown to have excellent fatigue and retention characteristics, although processing temperatures in excess of 750.degree. C. pose integration issues. For PZT, phase-pure thin films can be deposited at temperatures in the 450-650.degree. C. range, although Pt/PZT/Pt capacitors are known to suffer from poor fatigue and retention. In the prior art usage of previously known PZT materials, doping and/or oxide electrodes have been needed to produce satisfactory capacitor electrical properties.
Much of the previous work in the field that has established the feasibility of PZT and SBT for memory applications has focused on films that switch at 3V and above. Given the inexorable trends towards smaller circuit elements and lower operating voltages, it is extremely desirable to achieve high reliability and performance for thin films at low operating voltages, especially below 2V.
Low operating voltage requires a combination of adequately low coercive field (E.sub.c) and film thickness. SBT films have been shown to have low E.sub.c (.apprxeq.35 kV/cm) at thicknesses on the order of 140 nm, resulting in coercive voltages of 0.5V. SBT, however, is handicapped by a low value of switched ferroelectric polarization (P.sub.sw), typically less than 25 .mu.C/cm.sup.2. Furthermore, the thermal processing (800.degree. C.) required for improvement of thin film properties is considered severe and undesirable.
Several studies have presented thickness scaling data for PZT films as thin as .about.150 nm. See, for example, P. K. Larsen, G. J. M. Dormans, and P. J. Veldhoven, J. Appl. Phys., 76, (4), 1994; and A. K. Tagantsev, C. Pawlaczyk, K. Brooks, and N. Setter, Integrated Ferroelectrics, 4, (1), 1994. These studies have shown that as the film thickness is reduced, the coercive field increases and the polarization decreases. Such effects have been attributed to depletion and depolarizing phenomena (A. K. Tagantsev, C. Pawlaczyk, K. Brooks, M. Landivar, E. Colla and N. Setter, Integrated Ferroelectrics, 6, 309 (1995)).
The foregoing effects have been considered by the art to be intrinsic to thin film PZT, and thus low voltage and thickness scaling of PZT was discouraged.
The high ferroelectric polarization and low processing temperatures of PZT (compared to other materials) provide a compelling motivation to identify a form of PZT and a deposition process that allows scaling the material to low operating voltages.
It would therefore be a major advance in the art, and accordingly is an object of the present invention, to provide a form of PZT and a deposition process that allows scaling of the PZT material to low operating voltages.
It is another object of the invention to provide a PZT material that is scalable in lateral dimensions (i.e. dimensions parallel to the film surface) without incorporating in the material acceptor dopants or modifiers, e.g., Nb, Ta, La, Sr, Ca, etc.
It is a further object of the invention to provide a PZT material of the foregoing type, which is useful for ferroelectric capacitors over a broad range of thicknesses, particularly in the range of from about 20 to about 150 nanometers.
A still further object of the invention is to provide a PZT material that is useful for ferroelectric capacitors over a broad range of pulse lengths, particularly in the range of from about 5 nanoseconds to about 200 microseconds.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.